1. Field of the Invention
The present invention relates to a welding apparatus and more particularly to a monitoring system for measuring and recording a plurality of weld quality indicators.
2. Description of the Prior Art
A recurring problem in the welding art is control of a welding machine such that it provides welds of consistently high quality. Such welds are difficult to obtain because of numerous invisible and varying factors, such as the characteristic of the workpiece upon which welding is to be carried out, the condition of the welding electrodes, and fluctuations in welding current and electrode pressure. Therefore, it is necessary to test or monitor the welds in order to be assured that they meet certain standards of quality.
Non-destructive weld monitoring systems for monitoring the welding process of each weld and providing an instantaneous indication of weld quality are well known. A typical arrangement is disclosed in U.S. Pat. No. 3,708,648, granted to William C. Croucher et al., on Jan. 2, 1973, and assigned to the assignee of the present invention. In the Croucher et al. arrangement, a differential transformer is used to sense the linear movement of an extendable welding electrode, which movement is caused by the expansion, melting, and solidification of the weld in the workpiece which is gripped between the extendable electrode and a fixed electrode. First, the output of the differential transformer is sensed after the electrodes have been placed in operable relation with the workpiece, but prior to the beginning of a weld cycle. Second, the output of the transformer is sensed when a predetermined condition, such as maximum expansion, has been detected. By subtracting the first output value from the second output value, one can determine the travel of the extendable electrode relative to the position of the electrodes at the start of the weld cycle.
The Croucher et al. apparatus description employs electromechanical devices, relays, etc., to illustrate the sequence logic, although it was made clear that solid state switching is an alternative method for accomplishing the above. This device is limited because it stores and makes use of maximum values of electrode movement from null.
Other expansion using devices employ electromechanical sensors, geared electrical keys or clutches, which are used to preset a predetermined height so that the time to move from a zero point to the prescribed height is measured and used. The disadvantage inherent in these devices is the inaccuracy of the electromechanical systems especially with regard to quickly welded alloys, like aluminum, and the lack of versatility being only able to tell the time of a rise to one point on the rise slope.